1. Field of the Invention
The present invention relates to improvements of a distributed constant type LC noise filter.
2. Description of the Prior Art
There is known a noise filter which comprises an insulation substrate 10 of ceramic or other material, a swirl-like induction conductor 12 formed on the substrate at one side 10a and a ground conductor 14 formed on the same substrate at the other side 10b, as shown in FIGS. 16A and 16B.
This noise filter functions as an LC noise filter which provides an inductance L.sub.1 at the swirl-like induction conductor 12 and at the same time a distributed constant type electrostatic capacitance C between the swirl-like induction conductor 12 and the ground conductor 14.
However, such as LC noise noise filter raises the following problems:
(a) First Problem:
The noise filter could not provide an expected capacitance C between the induction conductor 12 and the ground conductor 14. Further, the noise filter could have its electrical characteristics only in a level inferior to those of a lumped constant type LC noise filter.
In particular, as the frequency of signals applied to the induction conductor 12 increases, the inductance thereat is changed from one level to another while decreasing the capacitance C between the induction conductor 12 and the ground conductor 14. This results in insufficient function in the LC noise filter.
(b) Second Problem:
If the noise filter is used in a signalling circuit, signals are applied to the opposite electrodes 16 and 18 of the induction conductor 12 to eliminate noise from these signals. When the frequency of signals to be applied increases, a line-to-line short-circuit is induced at the swirl-like induction conductor 12, as shown by arrow B in FIG. 16A. This line-to-line short-circuit impairs the function of the inductor conductor 12 as an inductor. Since such a line-to-line short-circuit is more frequently created as the frequency of signals to be applied increases, the prior art noise filter could not be used as a high-frequency noise filter.